This research proposal is designed to probe the role of cytoplasmic estrogen receptors in hormone-dependent and -independent rat mammary adenocarcinomas. Specifically we wish to ask if the receptors play a role in hormone transport into the cell. Further we wish to assay if the nuclear membrane specific binding site described by us in mature endometrium is also present in hormone-dependent breast tumor cells. More especially we wish to investigate whether it is absent in cells which contain cytoplasmic receptors but are hormone independent and do not respond by regression upon ovariectomy or hypophysectomy. In addition we wish to characterize the membrane site of binding, hopefully to isolate a binding protein and to characterize it chemically. In addition we hope to enquire whether this membrane fraction plays any role in control or modification of either DNA or (more likely) RNA synthesis.